This invention relates to electric motors, and more particularly to a permanent magnet rotor for an electric motor.
Numerous designs of permanent magnet rotors have been proposed in the past, particularly in recent years since advancements have been made in the quality of the magnets. In a typical rotor of this type a number of magnets are secured to the outer periphery of a cylindrical iron core. A problem that arises in the construction of this type of rotor has to do with securing the magnets to the rotor core. The securement has to be strong enough to withstand the centrifugal forces due to high speed rotation of the rotor; to withstand the angular forces due to abrupt changes in the angular velocity of the rotor; and to withstand axially directed forces that may arise if a motor is dropped, for example, during installation of the motor.
In some constructions, clamps and wedges have been provided to secure the magnets to the core, and in some instances adhesives have been used. In some other constructions a cover has been provided around the outside of the magnets. Such covers have taken the form of metal sleeves which, in some rotors, are heat shrunk over the magnets, or a binding made of a synthetic material which forms a covering.
The use of clamps and wedges is not considered satisfactory because this method is unduly expensive to manufacture and assemble. Balancing a rotor of this type may also be difficult. Adhesives are not acceptable in a motor which is filled with a liquid such as water because presently known adhesives will fail when submersed in water over a long period of time. Metal sleeves have the capability of restraining the magnets against centrifugal force, but prior art sleeves have not provided sufficient restraining force in the axial direction. Some rotors having outer sleeves have also included plates at the ends of the rotor, which may have the purpose of providing axial restraint.
It is therefore a general object of the present invention to provide an improved permanent magnet rotor for an electric motor, including a metal sleeve which holds the magnets against centrifugal, angular and axial forces.